1. Field of the Invention
The present invention generally relates to controlling the environmental conditions surrounding a device. More particularly, the present invention relates to a method and apparatus for providing a uniform air flow and temperature near a plurality of semiconductor devices during performance testing.
2. Description of the Invention Background
An integrated circuit is a solid state device in which electrical components and electrical connections between the components are incorporated into a solid matrix by the strategic placement of various conducting, semiconducting and insulating materials to form and encapsulate the desired circuit in the composite solid matrix. The development of the integrated circuit has led to the miniaturization of electronics by providing a strong matrix to support and protect fragile miniaturized components and connections and facilitating the placement of the electrical components in close proximity. The integrated circuit has also served to increase the reliability of electronic devices by the elimination of moving parts and fragile wiring and electrical connections.
Integrated circuits are typically constructed by depositing a series of individual layers of predetermined materials on a wafer shaped semiconductor substrate, or "wafer". The individual layers of the integrated circuit are, in turn, produced by a series of manufacturing steps. The precise characteristics of the layers, such as composition, thickness, surface quality, uniquely determine the electronic properties and the performance of the integrated circuit. Testing of the integrated circuits is performed throughout the manufacturing process in order to ensure that the circuits perform within the design specifications and to serve as a quality control indicator for the preceding manufacturing steps.
In addition to the short-term performance testing described above, longer term performance tests are run to provide life estimations and compensation factors for performance variations. The conditions used in performance testing generally vary from normal operation to extreme condition testing typically ranging from -50.degree. C. to 150.degree. C. Extreme condition testing, such as burn-in testing, is used to identify seriously defective conditions in the circuit that would lead to early lifetime failures, while long-term normal operation testing is directed to identifying systemic problems in the circuit.
It is desirable to perform the testing of the circuits in a manufacturing process time scale to prevent the testing from forming a bottleneck in the manufacturing process and to quickly identify changes in the manufacturing process. Therefore, the integrated circuit devices are generally tested in bulk quantities of 8 to 64 devices at a time depending upon the particular test. However, the bulk testing of the devices causes problems with maintaining uniform environmental test conditions near each device, which can lead to erroneous test results. Because the test results are used not only to provide an indication of the quality of the individual device, but to form a statistical sampling for the determination of process control conditions, erroneous test results can have a substantial impact on the cost effectiveness and the quality of the manufacturing process. Generally, the bulk device performance test procedures are considered to be reliable and the testing in control if the temperature variation of the devices is within .+-.4.degree. C.
The variation in the environmental conditions near the devices under test (DUTs) is important because the performance and failure of the device is a strong function of the device temperature.
The testing of the devices is normally performed within a test apparatus having a contained environment to allow the test conditions to be varied depending upon the particular performance test being run. However, the devices under test will generate heat due to resistive losses that may not be freely transferred away from the device resulting in an increased local environmental temperatures and operating temperatures of the devices under test. This type of variation can be quantified to some extent and has been compensated for in the prior art through the use of weighting factors. A second type of variation occurs as a result of nonuniform air flow patterns within the apparatus that produce device to device variations in the amount of heat transfer from the device to the environment resulting in device to device temperature and performance variations. The second type of variation is not as easily quantifiable, if at all, due to the contributing factors of nonuniform air flow, nonuniform environmental temperatures and actual variations in the performance of devices. It is, therefore, very difficult to assess the variations in performance due to the manufacturing process or to determine the precise performance rating for the device in the bulk testing apparatuses of the prior art. While specific data can be obtained by performing individual testing of the devices, individual device test methods are not sufficiently rapid to allow more than a small sampling of the devices to be made without introducing a severe bottleneck into the manufacturing process.
One attempt to provide an integrated circuit device performance test apparatus that addresses the device to device variation is provided by U.S. Pat. No. 5,359,285 to Hashinaga et al. The Hashinaga patent requires that the integrated circuit to be tested contain a temperature sensing device that is used in conjunction with the apparatus. The apparatus includes a control device that is connected to the temperature sensing device and an air flow and direction controller. The control device is connected through a temperature detection device to the individual devices and the temperatures of the individual devices are monitored by the control device. In response to the temperature data, the control device adjusts the direction and flow rate of the air in the apparatus through a series of ports. The feedback loop operates to maintain the temperature of the devices under test within a prescribed temperature range.
A number of practical problems are encountered when using the apparatus of the Hashinaga patent. While, the incorporation of a temperature sensor into the chip provides a more precise way to determine the temperature of the actual device, it may not be practical, desirable or cost effective to incorporate a temperature sensor into each device. The embodiments disclosed in the Hashinaga patent also envision a limited number of directional air flow nozzles servicing a large number of devices, which appears to present control problems considering that the control device will be monitoring all of the devices to control the direction of the air flow. Also, in order to service a number of devices the air flow must be directed substantially parallel to the devices under test which will result in different devices experiencing different air flow patterns and temperatures. The aforementioned problems can be somewhat overcome, for instance, by providing an individual air flow nozzle for each device under test. This results in a very complex and cost ineffective system requiring experienced oversight at all times and an extensive requalification of the baffling used to control the air flow every time the system is taken down for repairs or preventive maintenance. Even if these problems can be addressed satisfactorily, a basic problem remains in the system. The Hashinaga patent attempts to provide a method and apparatus that allows for the performance testing of the devices at a prescribed temperature attributing all of the variation in the device temperature to the variations in the apparatus. However, a portion of the device to device variation observed is actually attributable to variations in the manufacture of the device, which provides important data on quality control of the manufacturing process in addition to the performance of the individual device.
Thus, it is apparent that a need exists for an improved integrated circuit device performance testing apparatus which overcomes, among others, the above-discussed problems so as to provide more reliable data on the performance of the individual devices and manufacturing process in a cost effective and reliable manner.